The neem tree, a tropical evergreen, has been used for centuries as a source of pesticides to which insects have not developed a resistance. Various neem seed extracts, particularly the ones containing the hydrophilic, tetranortriterpenoid azadirachtin, are known to influence the feeding behavior, metamorphosis (insect growth regulating [IGR] effect), fecundity, and fitness of numerous insect species belonging to various orders.
Neem oil, containing azadirachtin, may be mechanically pressed from neem seeds in the cold by using oil presses or may be extracted using alcohols or other solvents using a Soxhlet apparatus. Small amounts of neem oil can be obtained by kneading neem seed powder by hand after adding some water (Schmutterer & Helip 1988). Thus the term `neem oil` has been used to describe a variety of materials containing a mixture of both hydrophilic and hydrophobic extractables. The variety of extraction methods and resultant variety in composition of neem oil has led to great confusion as to the true properties of "neem oil." Khan and Wassilew (1986) tested the effect of their "neem oil" (prepared by aqueous extraction of neem kernels) on 14 common fungi, including Trichophytonrubrum, T. violaceus, T. concentrichus, T. mentagrophytes, Epidermophyton floccosum, Miersporum citaneum, Scrophulariopsis brevicaulis, Geotrichum candidum and Fusarium sp and found that it did not inhibit fungal growth and, in fact, the neem oil itself actually contained several species of growing fungi. Yet an anonymous article (Anon. 1986) reported that "10% Neem oil diluted from its emulsifiable concentrate formulation" completely inhibited several species of fungi such as Aspergillus niger, Fusarium moniliforme, Macrophomina phaseolina and Drechslera rostrata. However, the specific details of this formulation were not provided.
Kahn et al. (1986) report that neem oil (source unidentified) showed no inhibitory affect on the growth of a variety of fungi. In fact, it is reported that the neem oil was contaminated with molds including Aspergillus niger and Aspergillus flavus. Similarly, Sharma et al. (1986) report that 3-5% neem seed oils (no method of preparation given) had no effect on the control of pod borer Heliothis armigera on chickpea, in Gujar et al. (1985) report that neem seed oil (no method of preparation given) had no effect on the desert locust Schistocerca gregaria.
Contrary to this, it has been reported that neem oil formulations prepared by expressing oil from the seeds or by extracting with aqueous solvents are effective insecticides and fungicides. It is reported that 10% neem oil (preparation unidentified) (Anon 1986) inhibited the growth of certain fungi in vitro. Dryer (1986) discloses that neem oil obtained by hand pressing in the cold was similar in the control of phytophagous arthropods to aqueous extracts; expeller-pressed oil was much less active.
Similarly, there are discrepancies in the literature as to the use of neem oil to control insects. Schmutterer and Hallpap (1986) showed that aqueous neem seed extracts are significantly superior to neem oil in repelling leaf mites (Scrobipalpa ergasina), leaf roller (Phycita melogenu) and leaf hopper (Jacobiella faciaina). Mansour et al. (1986) report that the pentane extract of neem seeds was much more effective at controlling the spider mite Tetranychus cinnabarinus than were ethanol or methanol extracts, but surprisingly, the pentane extract was less effective at controlling the mite, Phytoseiulus persimilis than were the ethanol or methanol extracts.
Yamasaki et al. showed that the tetranortriterpenoid salannin can be isolated from crude plant extracts, obtained from indian neem seeds which are known to be high in salannin content, using hexane. The biological activity of the salannin extract is reported to be feeding deterrency and growth inhibition when applied to chewing insects such as beetles and caterpillars.
Crude neem oil is not shelf-stable and can lose biocontrol activity. A solvent extraction method of U.S. Pat. No. 5,409,708, incorporated herein by reference, can be used to remove stability-reducing components from crude neem oil. This solvent-extraction method has the effect of reducing the cloud point of the crude neem oil, and increasing the shelf stability of the clarified product. However, the method requires a solvent-stripping step and an oil-fractionating step wherein certain waxes and fatty acids therein solidify and are filtered out. Moreover, the method significantly reduces the input neem oil volume and thereby creates a waste product stream that must be disposed of at significant expense to producers or consumers. According to the patent, when 35 pounds of crude neem oil were clarified by solvent extraction, 30 pounds of clarified oil and five pounds of neem wax were produced. Although both the clarified oil and the wax fractions are active, the clarified oil fraction is more effective for foliar applications, since the wax fraction exhibits higher phytotoxicity. Thus the wax fraction may find its most suitable use in formulations applied to dormant plants or non-plant material.
It would be desirable in the art, for both economic and other practical reasons, to produce a clarified neem oil having high shelf stability and low foliar phytotoxicity while minimizing the production of a neem wax byproduct.